A reciprocating engine to be employed in a motor vehicle, a motorcycle, an agricultural machine, a marine vessel or the like requires a crankshaft to extract power by converting reciprocating motions of pistons to rotational motion. There are two types of crankshafts: the type manufactured by die forging and the type manufactured by casting. Especially when high strength and high stiffness are required, die forged crankshafts (which will hereinafter be referred to as forged crankshafts) are often employed.
A forged crankshaft is generally produced by using a billet as a starting material. The billet has a circular or square cross section, and the cross sectional area is constant throughout the length. A method for producing a forged crankshaft includes a preforming step, a die forging step, and a trimming step. After the trimming step, a coining step may be performed if needed. Typically, the preforming step includes a rolling step and a bending step, and the die forging step includes a rough forging step and a finish forging step.
FIGS. 1A to 1F are schematic diagrams illustrating a conventional production process for producing a forged crankshaft. The crankshaft 1 illustrated in FIG. 1F is a four-cylinder eight-counterweight crankshaft to be mounted in a four-cylinder engine. The crankshaft 1 comprises five journals J1 to J5, four pins P1 to P4, a front part Fr, a flange Fl, and eight crank arms (hereinafter referred to simply as “arms”) A1 to A8. The eight arms A1 to A8 connect the journals J1 to J5 respectively to the pins P1 to P4. The eight arms A1 to A8 have counterweights (hereinafter referred to simply as “weights”) W1 to W8, respectively. The weights W1 to W8 are integrally formed with the arms A1 to A8, respectively.
In the following paragraphs, when the journals J1 to J5, the pins P1 to P4, the arms A1 to A8, and the weights W1 to W8 are each collectively referred to, a reference character “J” is used for the journals, a reference character “P” for the pins, a reference character “A” for the arms, and a reference character “W” for the weights.
According to the production method shown in FIGS. 1A to 1F, the forged crankshaft 1 is produced in the following manner. First, a billet 2 with a predetermined length as shown in FIG. 1A is heated in a heating furnace or a gas atmosphere furnace and then undergoes rolling. In the rolling step, the billet 2 is rolled and drawn by grooved rolls, for example, to distribute its volume in the longitudinal direction, whereby a rolled blank 3, which is an intermediate material, is formed (see FIG. 1B). Next, in the bending step, the rolled blank 3 is partially pressed in a direction perpendicular to the longitudinal direction to distribute its volume, whereby a bent blank 4, which is a secondary intermediate material, is formed (see FIG. 1C).
Next, in the rough forging step, the bent blank 4 is press forged by a pair of upper and lower dies, whereby a rough forged blank 5 is formed (see FIG. 1D). The rough forged blank 5 has a general shape of the crankshaft (final product). Then, in the finish forging step, the rough forged blank 5 is further press forged by a pair of upper and lower dies, whereby a finish forged blank 6 is formed (see FIG. 1E). The finish forged blank 6 has a shape in agreement with the shape of the crankshaft that is a final product. In the rough forging and the finish forging, excess material flows out from between the mutually opposed parting surfaces of the dies, thereby forming flash. Therefore, the rough forged blank 5 and the finish forged blank 6 have large flash B around the shape of the crankshaft.
In the trimming step, while the finish forged blank 6 with flash is held by a pair of dies, the flash is punched by a cutting die. Thereby, the flash B is removed from the finish forged blank 6. In this manner, a finish forged blank with no flash is obtained, and the finish forged blank has almost the same shape as the forged crankshaft 1 shown in FIG. 1F.
In the coining step, principal parts of the forged blank with no flash are slightly pressed by dies from above and below and shaped to have the correct size and shape of the final product. In this regard, the principal parts of the forged blank with no flash are, e.g., shaft parts such as the journals J, the pins P, the front part Fr and the flange Fl, and in some cases the arms A and the weights W. In this manner, the forged crankshaft 1 is produced.
The production process shown in FIGS. 1A to 1F is applicable not only for producing a 4-cylinder 8-counterweight crankshaft as illustrated in FIG. 1F but also for producing various other types of crankshafts. For example, the production process is applicable for producing a 4-cylinder 4-counterweight crankshaft.
In a 4-cylinder 4-counterweight crankshaft, some of the eight arms A have weights W integrated therewith. For example, the leading first arm A1, the trailing eighth arm A8, and the two central arms (the fourth arm A4 and the fifth arm A5) each have a weight W integrated therewith. The other arms, and specifically, the second, the third, the sixth and the seventh arms A2, A3, A6 and A7 have no weights, and these arms are oval. In the following paragraphs, such an arm with no weight will be sometimes referred to as “non-weight arm”.
Also, the same production process can be applied for producing crankshafts that are to be mounted in a 3-cylinder engine, an inline 6-cylinder engine, a V-type 6-cylinder engine, an 8-cylinder engine and the like. It is noted that, when adjustment of the placement angles of the pins is necessary, a twisting step is added after the trimming step.
In recent years, there has been a need for weight reduction of reciprocating engines, particularly those for motor vehicles, in order to improve the fuel economy. Accordingly, there is also an ever-increasing demand for weight reduction of crankshafts to be mounted in reciprocating engines.
A conventional way to reduce the weight of a forged crankshaft is providing a recessed portion in a pin-facing surface of an arm incorporating a weight. The recessed thin portion is formed by die forging, and therefore, the recessed thin portion extends in a direction perpendicular to the parting plane of the dies, that is, in a direction perpendicular to the pin decentering direction and reaches both side surfaces of the arm. This is disclosed in Japanese Patent Application Publication No. 2009-197929 (Patent Literature 1) and Japanese Patent Application Publication No. 2010-255834 (Patent Literature 2).
In the crankshaft disclosed in Patent Literature 1, the recessed thin portion extends in a direction perpendicular to the pin decentering direction and reaches both side surfaces of the arm. In the recessed portion, at least in a region that is less distant from the pin than the axis of the journal, the depth of the recess increases gradually with increasing distance from the pin and decreasing distance from the journal. Also, the bottom of the recessed portion is formed to be along the outer periphery of an imaginary cylinder. The imaginary cylinder extends from the joint surface of the pin and the arm (web) to the joint surface of the journal and the arm (web). According to Patent Literature 1, this allows for weight reduction without causing a reduction in the stiffness of the crankshaft.
In the crankshaft disclosed in Patent Literature 2, a thin portion is formed in a pin-facing surface of an arm, and the thin portion extends toward the journal to an imaginary line. The imaginary line is a straight line passing the axis of the journal in a region between the periphery of the thrust bearing of the pin and the periphery of the thrust bearing of the journal. The thin portion extends in a direction perpendicular to the parting plane of the dies, that is, in a direction perpendicular to the pin decentering direction, and reaches both side surfaces of the arm. According to Patent Literature 2, because of the thin portion, the arm bends when a reciprocating motion of a piston puts a load on the pin, and therefore, the stress is dispersed, thereby lengthening the life of the pin. Patent Literature 2 teaches that providing a recessed portion allows for a reduction in weight.
Another conventional way to reduce the weight of a forged crankshaft is making a hole by punching. This is disclosed in Japanese Patent Application Publication No. 2012-7726 (Patent Literature 3) and Japanese Patent Application Publication No. 2010-230027 (Patent Literature 4).
Patent Literatures 3 and 4 teach an arm having a hole made in the journal-facing surface and teach a method for producing a crankshaft with the arm. The hole of the arm is made to lie on a straight line connecting the axis of the journal and the axis of the pin (which will be hereinafter referred to as “arm centerline”), and the hole extends large and deep toward the pin. This arm is reduced in weight by the weight corresponding to the volume of the hole. The weight reduction of the arm leads to a weight reduction of the weight paired with the arm, thereby resulting in a reduction in weight of the whole forged crankshaft. In the region of the arm near the pin, the both side portions of the arm are thick, which ensures the stiffness (both torsional rigidity and flexural rigidity). The both side portions of the arm mean the surfaces at edges in the arm width direction (in the direction perpendicular to the pin decentering direction) and therearound.
Forming a recessed portion in the journal-facing surface of the arm while keeping the both side portions of the arm thick as described above ensures both weight reduction and stiffness.
It is, however, difficult to produce such a forged crankshaft with such arms having a unique shape by a conventional production method. The reason is as follows. When a recess is to be formed in the surface of an arm in the die forging step, the draft of the die will become a reverse draft at the site of the recess, and therefore the formed forged blank will not be able to be removed from the die.
To avoid such situations, in the production methods disclosed in Patent Literatures 3 and 4 are configured as follows. In the die forging step, the arm is shaped to be small with no recess formed in the surface of the arm, and after the trimming step, a punch is pushed into the surface of the arm to form a recess.
In the crankshaft shown in FIG. 1F, all of the arms A and the weights W incorporated therewith have the same shape. Practically, however, the arms A and the weights W integrated therewith may be different from one another in shape as needed. Japanese Patent Application Publication No. 2007-71227 (Patent Literature 5) and Japanese Patent Application Publication No. 2014-40856 (Patent Literature 6) disclose techniques for this.
Patent Literature 5 discloses a 4-cylinder 8-counterweight crankshaft including a flywheel disposed at an end. In the crankshaft, the arms incorporating a weight are different from one another in the thickness and the center of gravity of the arm and in the mass of the weight. Accordingly, it is possible to reduce the thicknesses of the arms that need to have low stiffness while ensuring the minimum necessary stiffness to each of the arms, thereby resulting in a reduction in weight.
Patent Literature 6 discloses a crankshaft for a multicylinder engine, the crankshaft including a flywheel disposed at an end. In the crankshaft, an arm that is less distant from the flywheel has higher flexural rigidity and higher torsional rigidity than an arm that is more distant from the flywheel. Also, it is preferred that the arms are different from one another in the flexural rigidity and in the torsional rigidity. Accordingly, it is possible to attain a reduction in weight while suppressing flexural vibration and torsional vibration.
In such a case in which the arm shape and the weight shape of each arm are different from those of any other arm, what portion of the arm needs to have high stiffness differs from arm to arm, depending on the shape. Specifically, an arm may need to have high stiffness near the pin, and another arm may need to have high stiffness near the journal.